1. Field of the Invention
The present invention relates to a rear projection screen, more particularly, to a rear projection scrren employing wax as a main component.
2. Description of the Prior Art
A rear projection screen (hereinafter, a rear projection screen or screens will be referred to as an "RPS") is an optical element on which image information is projected from the side opposite to the viewers and on which image information can be reproduced. It is used for optical display devices, e.g., reading devices for microfilms, or projection devices for movies.
RPS are well known and are made by employing various materials and methods. They can be classified into the following types:
1. RPS having a matted surface (hereinafter referred to as a "mat type screen"),
2. RPS having a layer containing dispersed light-scattering particles (hereinafter referred to as a "dispersion type screen").
3. RPS having a wax layer sandwiched between two transparent supports (hereinafter referred to as a "wax type screen"), and
4. RPS employing a polymeric material with a crystalline structure (hereinafter referred to as a "crystalline polymer type screen").
With respect to the optical properties required for such RPS, it is important that they have:
(1) LITTLE OR NO SCINTILLATION,
(2) A BROAD IMAGE REPRODUCTION RANGE,
(3) GOOD LIGHT-REDISTRIBUTION CAPABILITY (DIFFUSING CAPABILITY), AND
(4) HIGH RESOLVING POWER.
The term "scintillation" means the phenomenon that innumerable light-spots formed on the RPS glitter as a viewer moves his eyes along the RPS. Scintillation fatigues the eye viewers, and is a serious problem in microfilm viewing. One of the most serious problems in the optical properties of RPS is their scintillation.
The term "image reproduction range" is a characteristic value showing the density range which images projected on a screen can reproduce, and is defined as follows ##EQU1## This evaluation factor has hitherto been called "contrast", and the inventors consider this an important factor where continuous tone images are to be faithfully reproduced.
The image density range of light-sensitive photographic films is from about 3.0 to 4.0 or more, and thus includes the image density region of 2.5 to 3.0 which can be detected by the naked eye. However, the image reproduction range of commercially available RPS is 1.2 to 1.6, and hence commercially available RPS cannot faithfully reproduce image information from an original.
In order to increase the image reproduction range, RPS must have a high diffuse transmittance and a low diffuse reflectance. The former increases the maximum screen luminance, and the latter reduces the minimum screen luminance because reflected room light is lower, and the latter simultaneously increases maximum screen luminance because it lowers loss of light from the light source on the screen.
Ther term "light redistribution capability (diffusion capability)" ideally means that where image information is projected on an RPS, the image reproduction range of the screen does not vary, even if it is observed from any direction. In general, it is desired that lumininance distribution on the screen be uniform observed from any direction. In order to practically obtain the desired illuminance distribution, the diffusion capability of an RPS should be close to that of opal glass, in which incident light is scattered uniformly in all directions, or an RPS should be used together with an optical element which has the function that light is uniformly distibuted only the on areas to be observed.
In order to evaluate the "light redistribution property" of an RPS, the inventors have used the incident angle (.theta.1/2); the incident angle shows the angle at which the luminance is reduced to 1/2 of that when light is exposed at a right angle onto the RPS. For example, when .theta.1/2 is 25.degree., the luminance of the screen at 25.degree. is 1/2 the luminance in the case of right angle incident light.
The term "resolving power" means the number of lines per 1 mm resolved on the RPS. This value is decided on the basis of the resolving capacity of the naked eye, and is not less than the capacity of the naked eye.
The most serious defects of a mat type screen are high glare and a narrow image reproduction range. The reason for these defects is based on the fact that light-scattering is hardly caused on the matted surface of the screen, that is, at the surface where the refractive index difference between air and the screen is large. Further, since light-scattering occurs on only the surface of the screen, scratches on the matted surface are conspicuous and lower the commercial value of the screen. Since light-scattering occurs on only surface of the screen, it is difficult to provide a desired diffusion capability to the screen.
The most serious defect of dispersion type screens is high scintillation. In order to decrease the scintillation of the screen, it has been considered to increase the number of light-scattering particles per unit area (i.e., particle density) by reducing the particle size and to reduce the difference in refractive index between the binder and the particles therein (see Japanese Patent Application (OPI) 2127/71 and U.S. Pat. No. 3,712,707); while scintillation can be decreased, resolving power is reduced, diffuse reflectance increased and diffuse transmittance decreased, whereby the image reproduction range is markedly lowered. In fact, since the resolving power becomes substantially the same as in the thickness of the diffusion layer in the system, the thickness of the diffusion layer must be limited to about 100 microns to obtain a resolving power of about 10 lines/mm, which is close to the resolving capacity of the naked eye. It is therefore impossible to overcome the scintillation problem in such a system because the factors in lowering scintillation and increasing the reproduction range and resolving power are opposed. Dispersion type screens can be prepared by uniformly coating a coating composition containing dispersed light-scattering particles onto a transparent support such as an acrylic resin plate or a glass plate and then drying. However, it is not easy to uniformly coat a layer having a thickness of 100 microns or less and an area of at least 30 cm .times. 30 cm on a hard transparent support, and high precision machinery is needed.
On the other hand, wax type screens have excellent optical properties as compared with the other type of screens. Particularly, they have substantially no scintillation and have a broad image reproduction range. It is surprising that even if the thickness of the light-scattering layer in a wax type screen is 1 to 2 mm, the resolving power is not reduced, while it is difficult to make the thickness of the light-scattering layer in a dispersion type screen greater than 0.1 mm. The surprising particular optical properties of wax type screens depend on the physical properties of the wax itself.
Wax can have complicated crystal shapes (e.g., twig shaped, needle shaped, plate shaped, block shaped, etc.).Depending on the crystallization conditions, there is a small density difference (refractive index) between crystalline regions or between crystalline areas and non-crystalline areas, and the refractive index continuously changes at the interface thereof, whereby incident light is passed through a complicated density zone in the light diffusion layer with multi-refraction and multi-scattering without overall reflection, so that light is not reflected to the incident direction.
However, a wax per se cannot be molded into a thin sheet because it is very soft and fragile, and so it cannot be used unless it is sandwiched between two transparent supports. It is desired in RPS construction that the number of the surfaces be as low as possible between a light diffusion layer and a transparent support, because loss of light is caused at the interface between substances having different refractive indices. Accordingly, the structure of a conventional wax type screen makes the production of such a screen difficult and inherently lowers the optical properties thereof. Further, such a screen has the serious disadvantage that the wax layer is easily stripped from the support or cracks with the passage of time because adhesion to the transparent support is weak due to the chemical inactivity and fragile nature of the wax. Therefore, though wax type screens have remarkably excellent optical properties, they have not been used on a commercial scale.
A crystalline polymer type RPS has better optical properties than a dispersion type RPS, but has worse scintillation and a poorer image reproduction range than a wax type RPS. This is due to the fact that as molecular weight increases the length of the molecule is longer and the viscosity of the molten substance is higher, whereby a complicated crystalline structure, such as in wax, cannot be formed by crystallization, rather, a micro-level structure of crystalline spherulites is formed, which degrades the optical properties of the screen.
In order to overcome this defect, other techniques for deforming such a crystalline spherulite structure are necessary, and such are disclosed in Japanese Patent Publication 19,257/73 and U.S. Pat. Nos. 3,573,141, 3,591,253 and 3,682,530).
High molecular weight substances have the advantages of film-forming capability and a more excellent mechanical strength than wax, but molding such substances into sheets costs more because larger apparatus for rolling, extrusion or injection molding are needed and production costs increase.